Ostracod faunas in the Purbeck and Wealden of England

The occurrence and abundance of ostracods found in 98 subdivisions (Faunicycles) of late Jurassic to early Cretaceous age in the English Purbeck and Wealden are summarised. Most of the taxa found are illustrated from holotype or other material and details of their ranges and abundance given in relation to each faunicycle. The characters of the ostracod assemblages and zones are described. Three new species (Cypridea brendae, C. hispida and Eoparacypris edmundsi); also two new subspecies (Cypridea setina pelota and C. tuberculata dorsiclavaia) are described and figured. A lectotype for Palaeocytheridea pellucida is selected and figured.

Anderson's figures form the lower part of this formation and of the Fairlight division as used informally by Morter (1984); those labelled Ashdown Sand form the Ashdown Sand division (Morter, 1984); both are variable lithologically and are not considered formal, laterally persisent lithostratigraphic units." Dr. R. H. Bate comments "This paper summarises the outstanding contribution made by the late F. W. Anderson to modern ostracod research, particularly the recognition of the importance of ostracods in biostratigraphy. His interpretation of the salinity tolerance of some ostracods in the Purbeck-Wealden is, however, at variance with that of other ostracod workers especially for the genusDurwinuZu -presently found living in freshwater. If this environment was also occupied by this taxa at that time, there would be a change of emphasis from the 'S' phase to the 'C' phase for the genus." The work was nearly completed at the time of Dr. Anderson's death in 1982. The manuscript was further prepared for the press by Dr. H. C. Ivimey-Cook of the British Geological Survey (B.G.S.) and is published with the approval of the Director, B. G. S. (N.E.R.C.). Communications regarding it should be addressed to the Biostratigraphy Research Group, B.G.S. Keyworth, Nottingham NG12 SGG. The B.G.S. holds many of Dr. Anderson's original diagrams and notes, as well as range charts which are too large and complex to be published, (see Appendix herein).
On his behalf we would also like to acknowledge the skill of Mrs. B. E. Coleman who took most of the photographs of these ostracods on a Cambridge Model 2A Stereoscan. Some extra photographs were taken by Mr. I. P. Wilkinson and Dr. J. Whittaker (B.M.(N.H.)) to complete the plates.
In view of the size and format, the Editor has allowed some departure from the usual instructions to authors in its presentation.
The cost of publication has been partly covered by the receipt of a grant from the British Geological Survey.

INTRODUCTION
The Purbeck and Wealden beds in England span the Jurassic to Cretaceous boundary and present a range of sedimentary environments from hypersaline through varying degrees of salinity to virtually freshwater. The sediments range from clays, often sphaerosideritic, through mudstones, limestones and siltstones to coarse, commonly fluvial sandstones (Allen, 1976;1981).
Geographically, two distinct areas can be recognised (Fig. 1). To the east, a steadily subsiding basin occupied what is now the Weald of Sussex and Kent and part of east Hampshire; to the west, a lagoon covered west Hampshire, Dorset, the Isle of Wight, and parts of Buckinghamshire, Berkshire and Wiltshire. Everywhere the water remained shallow, and particularly in the Wealden, plant beds with lignite are common. The fauna includes bivalves, gastropods, foraminifera, ostracods, fish, reptiles and primitive mammals.
The most abundant and widely distributed fossils are ostracods which afford the most suitable material for stratigraphical correlation. Under the stress of a shallow water environment varying in salinity, they produced a large number of species and varieties, some of which had a limited salinity tolerance with consequent variation in their numbers in any particular assemblage. Thus, there developed what the writer has referred to as Faunicycles, i.e. an alternation of ostracod faunas with a high salinity tolerance with those having a preference for fresher water. Several faunicycles are found within each characteristic faunal assemblage (Figs. 2 and 3) and these allow much closer stratigraphical correlation than is possible using the faunal assemblage or zonal distribution alone. This is important in view of the difficulty of lithological comparison in such a varied sequence of sediments.
The earliest zonal division of the Purbeck beds by means of the ostracod faunas was that of Lye11 (1855) Fig. 1. Sketch map of the palaeogeography of the Purbeck with schematic isopachytes at 50m intervals. A 'western basin', centred in the English Channel south of the Isle of Wight, received sediment dominantly from the west. A subsidiary depression was present in Lyme Bay. In the north, the 'River Chiltern' separated an area of shallows south-east of Swindon from the Aylesbury area and the southern edge of the London Platform. Sediment also accumulated in an eastern basin in the Weald of Sussex and Kent. This was separated from the western basin by the Central Ridge (Bognor Swell of Allen, 1976) which extended from the Vale of Wardour, across the Hampshire Basin north-east of the Isle of Wight and to the south of the South Downs. who utilised species described by Fitton (1836) and Forbes (1855). The systematic work of T. R. Jones between 1878 and 1888, however, was a great step forward, and in 1885 Jones was able to list seven species from the Lower Purbeck, 13 from the Middle Purbeck and eight from the Upper Purbeck. Then followed a period of inactivity until the writer began work on the Dorset sections of the Purbeck beds in 1928, followed by Sylvester-Bradley (1941 b) in Dorset and Wiltshire, by W. Heap, an enthusiastic amateur geologist resident in Swanage, and by Barker (1966 a and b;1975) in Dorset.
Knowledge of the succession of ostracod faunas, particularly in the Weald, has been greatly increased by boreholes put down during the search for coal in Kent (Lamplugh, Kitchin & Pringle, 1923), for oil and gas in Hampshire and the Weald (Falcon & Kent, 1960), for gypsum in Sussex (Howitt, 1964) and as part of the Geological Survey six-inch mapping and exploratory drilling programme in Surrey and Sussex. A list of some of the diagrams now deposited with B.G.S., giving additional details of some of these faunal sequences and their correlations is given in the Appendix to this paper.

GENERAL STRATIGRAPHY PURBECK
Except in marginal areas such as Swindon and Aylesbury, where there is an alternation between Purbeck and Portland-type sediments, the transition from one lithology to the other is abrupt. After a relatively brief period at the beginning of the Purbeck, evaporitic environments prevailed in both the eastern and the western areas ( Fig. 1) giving rise to beds of gypsum and/or anhydrite with mudstones and algal limestones (Fig. 2). Then followed a series of Lower Purbeck mudstones and limestones, the Adit and Ice-House limestones of the Weald (eastern basin) and the Cypris Freestones of Dorset, culminating in the Dunkshaws Limestone in the east and its equivalent, the Hard Cockle Limestone (part of the Hard Cockle Beds) in the west. Similar sedimentary conditions prevailed throughout the remainder of the Lower Purbeck Soft Cockle Beds, continuing up into the Middle Purbeck as the Marly Freshwater and Cherty Freshwater Beds of Dorset. This sequence was interrupted by a marine episode which produced thick shell beds throughout the area (the so-called Cinder Beds) which are locally rich in oysters in the Isle of Purbeck. Though the area appears to have been flooded with sea water, the Cinder Beds are not known to extend beyond the area of deposition of the underlying less marine sediments and they show no evidence of a general discontinuity at their base. Their characteristic ostracod, Galliaecytheridea postsinuata, had already appeared in small numbers below the Cinder Beds and is also present in the beds immediately above. Moreover, when conditions again became more brackish, the pre-Cinder Beds ostracod fauna was re-established with no major change. Probably this more saline episode, though dramatic, was of short duration in view of the relatively small amount of interstitial sediment binding the oyster shells. However, both in the east and west it took some time to revert entirely to earlier conditions, hence the term Intermarine Beds for the sediments that followed in Dorset. The next unit is the Scallop Limestone, part of the Scallop Beds of Dorset, followed by shales-with-'beef' (layers of fibrous calcite).
The base of the Upper Purbeck in Dorset is marked by the Upper Broken Shell Limestone which is succeeded by shales and limestones apparently laid down in fresher water and containing beds largely composed of Viviparus these are here re-named as a 'phase'. Above the Henfield phase (Figs. 5 and 6) are 300m + of sandstones, silts and clays with occasional thin 'Paludina' limestones (Sussex Marble). In Fig. 6, the sequences in the central Weald are compared with those towards the northern and southern margins. Parts of the sequences are shown in greater detail to clarify the variations in the lithological and faunicycle successions. In the south-east, the clays are frequently sideritic and there is a lack of coarse sediment near the southern margin. Fig. 6 also shows that the preserved thickness of sediment representing a faunicycle may be very variable.

THE FAUNICYCLE
The sequence of ostracod faunas in Purbeck and Wealden strata shows that there is repeated alternation between faunas dominated by species of Cypridea (the C-phase) and those by species belonging to genera other than Cypridea, some of which clearly have a more saline provenance (the S-phase). These two phases appear to be antipathetic, from which it is assumed that salinity was an important factor in determining their composition. Thus, the S-phases are believed to represent periods of higher salinity, perhaps resulting from reduced rainfall, but in the Purbeck and Wealden, these probably never reaching true marine conditions. Conversely, the less saline waters of the C-phase were probably never completely fresh. The combination of an S-phase with the succeeding C-phase fauna represents a Faunicycle, 98 of which have so far been recognised ( Figs. 3 and 4). Faunicycles form a basis for correlation throughout the Purbeck and Wealden of southern England which is more detailed than is possible by other means. There are, however, some complications. For example, where it has been possible to study a Faunicycle in detailin a uniform lithology and at close intervals (see Anderson & Bazley, 1971, figs. 6 and 7)it is evident that the transition from the S-phase to the C-phase assemblages is anything but uniform, consisting rather of a series of minor rhythms. Furthermore, the salinity tolerance of S-phase genera varied greatly, from those like Macrodentina that clearly favoured a more saline environment, to the darwinulids that are generally most abundant in the middle range of a Faunicycle. In the Purbeck especially, extreme lithological variation affected the sequence of faunal changes. There are 'Dirt Beds', probably fossil soils, representing near terrestrial conditions; evaporites, generally beds of gypsum or anhydrite; and layers of 'beef' (fibrous calcite'probably resulting from penecontemporaneous pre-consolidation crystallisation. In general, however, there appears to have been a trend from the more saline waters of the Lower Purbeck, where the C-phase species in a faunicycle may represent only a small percentage of the total fauna, to the Upper Purbeck and Wealden, where they generally account for at least 50%. This change is briefly interrupted at times by a return to more saline conditions as in the Cinder Beds Faunicycle of the Purbeck (97% S-phase species) and in the Henfield Faunicycle of the Weald Clay (80% S-phase species).

SALINITY
Since the concept of a Faunicycle is based on the assumption that some, if not all, ostracods exhibit a salinity preference, it is essential to attempt some kind of estimate as to what this salinity range is. For the Purbeck-Wealden faunas, this has proved particularly difficult although Morter (1 984) has suggested salinity ranges for certain bivalves and gastropods. It is easy to demonstrate by association that some genera occurred in more saline water than others, but to establish the degree of salinity is another matter. An arbitrary grouping, therefore, has been adopted according to which all the species of the genus Cypridea are classed as C-phase forms (i.e. nearer to freshwater), and the species of all other genera as S-phase forms which are assumed to indicate marine or brackish-water conditions.
It was at one time thought that the Purbeck-Wealden deposits were laid down in a predominantly freshwater environment and therefore that the non-terrestial fauna, including the ostracods, was freshwater and presumably lacustrine. These deposits, however, contain a variety of taxa some of which may indicate proximity to land but not necessarily preservation in freshwater deposits (e.g. mammal, crocodile, turtle and dinosaur remains, the insect and isopod beds and the occurrence of gastropods such as Physa, Valvata  Ostracods are the most abundant fossils in the Purbeck and Wealden and are found in association with all these other forms. It could be expected, therefore, that some at least were marine, and some belong to genera already established earlier in the Jurassic. From the Fyledal Clay (Kimmeridgian-Portlandian) Christensen (1 968) listed in the Portlandian. Bate t (1965) recorded Darwinula, Limnocythere, Bisulcocypris, Theriosynoecum and Timiriasevia from the Bathonian of Oxfordshire and he regarded these as freshwater forms despite their association with a marine fauna.
The determination of 6I3C ratios by Allen & Keith (1 965), though as yet inconclusive, supported the suggestion that some at least of the Purbeck-Wealden ostracods were marine forms. Samples (of calcitic shells and some limestones) which gave a 6lSC value of -2 or more and are therefore thought to be marine may also contain ostracods such as Macrodentina, Procytheropteron, Galliaecytheridea, Eoparacypris, Bisulcocypris, Scabriculocypris, Timiriasevia and Rhinocypris.
Association with marine genera in earlier parts of the Jurassic does not necessarily mean that Purbeck-Wealden ostracod genera were marine, but neither does an association in the Tertiary with freshwater forms indicate the contrary. If such genera as Fabanella, Mantelliana and Theriosynoecum were freshwater, they must have been either fluviatile or lacustrine. If they were freshwater, this should be indicated by their geographical distribution (i.e. they should be more abundant at or near the effluents), but this has not been established. Fabanella is abundant in the Lower Purbeck, and Theriosynoecum in the Upper Weald Clay in all areas examined, not only in certain marginal areas. If lacustrine, they should be associated with freshwater molluscs such as Unio. Instead, the association is with Corbula and Liostrea. Although the S-phases of faunicycles above the Cinder Beds still contain examples of such genera as Macrodentina and some Liostrea, they were relatively less important, and species of Cypridea became more abundant and varied. In the Wealden it is probable that S-phase brackish-water conditions occurred intermittently for example in the Wadhurst Clay, in the Henfield phase at the base of the Weald Clay and, in particular, near the top of the Weald Clay, where Fabanella reappears and Theriosynoecum is very abundantpossibly heralding the marine transgression of the Lower Greensand. +Bate interpreted the freshwater ostracods as having been brought into a marine environment by rivers but they could also have been reworked and redeposited (Ed.).

FAUNAL ASSEMBLAGES
The rate of evolution of the Purbeck-Wealden ostracods varied considerably. New forms of Cypridea were constantly appearing, whereas evolution in the S-phase genera was much slower and some species continued throughout, virtually unchanged. This appears to be generally the case with most animals and the production of new species can be correlated with stress conditions such as those associated with shallow water and rapidly changing environments. Changes in the composition of the ostracod faunas in the Purbeck-Wealden are superimposed on the variations due to salinity and sedimentary changes. Thus, the divisions determined by sedimentation, salinity and evolutionary changes coincide only rarely. There does, however, appear to have been some recognisable changes in the character of the assemblage about every seven faunicycles (see Figs. 3 and 4), so that the evolutionary rate of the genus Cypridea at least appears to have been constant.
Assemblage 1 includes species found also in the Upper Portland beds. This assemblage is followed by 13 others after which, in Assemblage 15, the approach to more saline conditions becomes evident. In Anderson (1 973) the Purbeck-Wealden faunas were divided into 10 Assemblages, but as more evidence has become available, this is now considered to be inadequate and to be too much influenced by changes in sedimentation.

SEQUENCE OF OSTRACOD FAUNAS
Locality data are denoted bv abbreviations that can be identified from the list on pages 38-40. To show the extent of the change in the ostracod faunas between the Portland Beds and Assemblage 1 of the Purbeck, the species with an asterisk (*) are those that continue into overlying beds and the species with a dagger (t) are first recorded in that unit. Where authors of species are not given in this section, they are given in "Notes on the Species" which also gives a reference to a figure in Plates 1-12 for species from the Purbeck and Wealden.

Purbeck Beds
In the following account, the number of ostracod specimens used to determine the fauna of each faunicycle is recorded so that the significance of the percentages quoted for the species within the fauna can be assessed. Poorly preserved material or ostracods of uncertain horizon are omitted. The latter represent almost half of the total specimens (about 400.000) recovered. Fig. 2 tabulates the main lithostratigraphical units and Assemblages 1-15, relating these also to the faunicycles and zones based in species of Cypridea. i.e. it shows maxima-minima in the occurrence of species. Fig. 5 illustrates the inferred relationships of some of the species of Cypridea and also indicates the dominant species at various times and relates these to the lithostratigraphical units. Examples of the real thicknesses of these units are shown in Fig. 6.

ASSEMBLAGE 1
Represents the transition from Portland to Purbeck faunas. As seen at Swindon and in the Aylesbury area. S-phase genera such as Macrodentiria and Wolburgia are gradually superceded by Fabanella and Marirelliana and this may indicate a reduction in salinity. Four faunicycles have been recognised: Faunicycle 1, Quainton. 5485 specimens from A ! and sw.
S-phase species represent 999hof the fauna: Eocytheropteron eusarca Eoparacypris weedonensis . . Most of this cycle consists of evaporite deposits and few ostracods were seen. The fauna appears to be similar to that of the previous cycle.

ASSEMBLAGE 2
Species of Macrodentina are no longer seen; instead, Fabanella boloniensis is the dominant form (46%). C-phase species are more abundant, forming up to 25% of the assemblage, with Cypridea dunkeri as the dominant species.
This faunicycle follows the main evaporite phase and the ostracod fauna was not fully re-established. S-phase species, mainly F. boloniensis and Mantelliana purbeckensis, represent 75% of the fauna. The C-phase species are as before: Cypridea dunkeri, C. primaeva and C. tumescens.
C-phase species are mainly C. tumescens and C . dunkeri. Cypridea granulosa protogranulosa, the earliest subspecies of C. granulosa, appears in this faunicycle but is very scarce. Salinity as before, S-phase species represent 85% of the fauna, mostly F. boloniensis (83%).
The C-phase species are mainly C . dunkeri and C . peltoides. Cypridea peltoides eurygaster is a new subspecies which appears in this cycle but is rare.
Of the C-phase species, 26% are C . peltoides.

ASSEMBLAGE 3
In this assemblage C-phase species for the first time, in the Mountfield Faunicycle, represent more than 50% of the ostracod fauna. The dominant forms are Cypridea dunkeri and C . granulosa. In the S-phase, F. boloniensis is still the dominant species.
Of the C-phase species, C. dunkeri is the dominant form with C . tumescens.
C-phase species are now more abundant (39%). The dominant species is C. peltoides (29%); C . sagena is a new form whilst C. granulosa is still rare.
Of the C-phase species, Cypridea granulosa is now the dominant species (34%), with C . dunkeri (9%) and C . sugena (7%). New forms appearing are Cypridea lata and C . varians. From this cycle, up to and including the Royal Faunicycle, C . granulosa is the most abundant and characteristic species.
The base of the Goldspur Faunicycle marks the base of the Middle Purbeck Beds, but in general the fauna is not very different from that of the previous cycle. Of the C-phase species, C . granulosa (42%) is dominant. For the first time, the subspecies C. granulosa granulosa is the most abundant subspecies (26%). C . delicatula represents 7% of the fauna, C . dunkeri (4%) and C . sagena (4%). Faunicycle 18, Swanage. 4564 specimens from Br, CD, DB, Fa, GD, Hd, Ki, Mo, NP and Wa.
S-phase species represent 44% of the fauna. The dominant species are F. boloniensis (14%),D. leguminella (14%) and Eoparacypris weedonensis. As the salinity decreases, darwinulids become more important in the S-phase and F. boloniensis less so.

ASSEMBLAGE 4
Cypridea granulosa reaches its maximum abundance as the dominant C-phase species in this Assemblage and a new subspecies, C. granulosa fasciculata, appears which eventually becomes the commonest form of the species.
There appears to be a steady progression towards a lqss saline environment, this is only briefly interrupted by the Cinder Beds episode.
In the S-phases, darwinulids become increasingly abundant.
Of the C-phase species, C. granulosa is the dominant form (33%). The subspecies C . granulosa fmciculata appears for the first time but is not abundant. There are several new forms: Cypridea swanagensis, C . dolabrata, C . inaequalis, C . penshurstensis, C . aemulans nom. nov. (see later) and the important species, C. posticalis, which has only been found in this and the following faunicycle.
S-phase species represent 97% of the fauna with Galliaecytheridea postsinuata (66%) as the dominant form, with Klieana dictyota, D . leguminella and other species. This unit is an ostreid rich shell bed usually about 3 m thick. In Dorset, it is almost entirely composed of shells but in the Weald there is a muddy matrix.
C-phase species are much reduced and are mainly C. granulosa fasciculata. The species Cypridea altissima is present but this may only be an unornamented form of C. granulosa.
This faunicycle represents a dramatic increase in salinity which, however, need not have been of long duration. There is little overall disturbance in the composition of the C-phase faunas and they become reestablished in the succeeding cycle.
Of the C-phase species, C. granulosa is dominant (37%) is the dominant form. Cypridea amisia is a new form at this level.
Faunicycle 25, Royal. 55 specimens from Br, CD, DB, Hd and Mo? provide only a sparse fauna.

ASSEMBLAGE 5
In this assemblage, Cypridea granulosa, though present, is no longer the dominant C-phase species, being replaced by Cypridea amisia and C . vidrana. In the S-phase, Fabanella boloniensis has been largely replaced by Macrodentina mediostricta .

ASSEMBLAGE 6
This assemblage is marked by the increasing dominance of such characteristic Upper Purbeck species as Cypridea setina, C. propunctata and C. wicheri.
S-phase species represent 36% of the fauna with M . mediostricta as the dominant form (20%) and darwinulids.
This cycle marks the base of the Upper Purbeck during which C. setina and C. propunctatu become the most typical species.  .S-phase species represent 14% of the fauna and are mostly darwinulids (1 0%).

ASSEMBLAGE 7
This fauna is essentially a continuation of Assemblage 6 but with some new elements such as the remarkable species Cypridea morula (Pl. 8, fig. 16). Cypridea dolabrata and C . wicheri both develop a number of subspecies and C. brevirostrata becomes abundant. In the upper part of this assemblage the typical Wealden form C. tuberculata becomes the dominant C-phase species. Although the Hastings Faunicycle (41) has been previously taken as the base of the Wealden, the transition from the Purbeck is gradual and it is only in the Maresfield and Hythe Faunicycles (45-46) that typical Wealden faunas become firmly established. There is also a change in the ornament pattern of C. wicheri in the Bexhill Faunicycle (42) and this could well be taken as indicating a transition from Purbeck to Wealden faunas.
Of the C-phase species, C. brevirostrata is the dominant form (41%), with C. wicheri (22%), C . setina (18%) and the scarce but characteristic C. tuberculata adjuncta. There are also a few early examples of forms comparable with C. bispinosa, C. menevensis, C . paulsgrovensis and C . recta. Anderson (1962) took this Faunicycle to be the earliest of the Wealden but this was no more than a convention. There is neither a distinct faunal nor a lithological change at this level. Faunicycle 42, Bexhill. 1606 specimens from Ar, CD, He, Ki, Pe, Po and Wa.
No S-phase species seen.

ASSEMBLAGE 8
This ostracod assemblage is found in the lower part of the Wadhurst Clay. The S-phases are dominated by Theriosynoecum alleni and Rhinocypris jurassica . The common C-phase species are C. laevigata and C . tuberculata . C-phase species include Cypridea blackbushensis (26%) a new arrival, C. paulsgrovensis (24%), C . westfieldensis another new form, C. menevensis and C . laevigata. C. melvillei was seen but was very scarce.

ASSEMBLAGE 9
This assemblage is present in the upper part of the Wadhurst Clay. The S-phase species in this assemblage show little change, but in the C-phase, the previously common species C. paulsgrovensis, C. laevigata and C. tuberculata are replaced by C. melvillei, C. recta, C. aculeata and C. bispinosa. Cypridea arenosa, with some subspecies of C. laevigata and C. aculeata are characteristic.

ASSEMBLAGE 10
The fauna is little changed. C-phase faunas consist largely of C. bispinosa, C. recta and C. tuberculata, and the S-phase faunas of T. alleni with darwinulids. The subspecies C. recta tillsdenensis is an important component of this assemblage. In the upper part of the assemblage, the faunas are sparse and known from very few localities so that details given for each Faunicycle are unreliable.
S-phase species represent 2% of the fauna and are mostly darwinulids.
S-phase species represent 9% of the fauna and all are darwinulids.
S-phase species represent 8% of the fauna and are mostly darwinulids.

ASSEMBLAGE 11
The basal part of the Weald Clay (Horsham phase) contains a distinctive assemblage completely dominated by C. dorsispinata in the C-phase and with Miocytheridea henfieldensis and Theriosynoecum fittoni as the S-phase forms. Whatever the reason for these impoverished faunas and the disappearance of most of the previously existing species, the base of the Weald Clay is marked by a profound faunal change. Although at first ostracods are scarce, they soon become abundant although relatively few species are present. In Anderson (1967) the terms Horsham Group and Henfield Group were used to refer to the ostracod assemblages in the Lower Weald Clay. In view of the lithological connotation now attributed to the word 'Group', these are re-named as 'phases'.
S-phase species are represented by five specimens. All are Darwinula oblonga.
C-phase species are represented by one specimen of C. dorsispinata.  C-phase species are C. clavata (39%), C. tuberculata and C. valdensis.

ASSEMBLAGE 14
In the Upper Weald Clay, the C-phase fauna is largely made up of C. clavata and its subspecies, with C. tuberculata and C. valdensis. Important species are C. bogdenensis which is typical of the Gillmans and Ditchling Faunicycles, and C. rotundata which is a common form in this assemblage. In the S-phases, T. fittoni eventually replaces M . henfieldensis as the dominant species. In the Atherfield and Woodhatch Faunicycles, there is a marked increase in salinity. C-phase species are C. tuberculata (50%) and C.

ASSEMBLAGE 15
In the highest part of the Weald Clay and the transition beds to the Lower Greensand, the salinity was high throughout. Cypridea spinigera is a typical C-phase species and Theriosynoecum fittoni is by far the most abundant ostracod.

Cypridea helenae
Description. This appears to be related to Cypridea marina, but with much reduced ornament and a more limited stratigraphical distribution. There are usually a number of small auxiliary tubercles particularly near the anterior end but sometimes also in the lumbar area. Because of its limited vertical range this species is a useful stratigraphical index.
Subspecies bellatula Anderson,197 1 Eastbourne Faunicycle (Pl. 5 , fig. 6) Subspecies camelodes Anderson, 1962 Tyneham Faunicycle (Pl. 6, fig. 18) Subspecies deburghi Anderson,197 1 Hastings to Bexhill Faunicycles (Pl. 6, fig. 7) Subspecies dotica Anderson, 1962 Tisbury to Durdle Faunicycles (Pl. 6, fig. 12) Subspecies erumna Anderson, 1962 Mupes to Battle Faunicycles (Pl. 6, fig. 6) Subspecies florida Anderson, 1971 Bexhill to Eastbourne Faunicycles (Pl. 6, fig. 17) Subspecies rectidorsata Sylvester-Bradley, 1949 Studland to Battle Faunicycles (Pl. 6, fig. 14) Subspecies sefina (Anderson, 1939) Lulworth to Battle Faunicycles (Pl. 5, fig. 9) Subspecies pelota nov. Description. This subspecies is the last of the C. setina stock. In outline it is nearest to the subspecies rectidorsata, but is relatively shorter and higher and the beak and notch are barely visible. Distribution. Cypridea setina pelota has been recognised with certainty only from the Kingsclere Borehole not far above the base of the Wadhurst Clay, but a few ostracods from this horizon in the Chilcomb Down Borehole may also be of the subspecies. Horsham phase, C . dorsispinata Zone. Weald Clay. Diagnosis. Carapace oblong-ovate in lateral view. Rostrum medium sized, alveolus medium sized, cyathus cupate?. Shell surface covered with medium sized punctations and ornamented with sharp tubercles or blunt spines over the whole surface. The 3 lumbar tubercles are conspicuous and usually larger than the rest. The left valve is the larger. Description. The ornament pattern in this subspecies is similar to that generally found in C . tuberculata but the large lumbar tubercles are distinctive. No individuals have yet been found with a well preserved cyathus. Distribution. This form is characteristic of the lower part of the Weald Clay (Horsham phase). It has been found in the Warlingham Borehole (Hackhurst Faunicycle), the Warninglid Borehole (Redhill and Hackhurst Faunicycles). In the Henfield phase above, the 3 lumbar tubercles are less conspicuous and both at the Southwater Brick Pit and at Warninglid a variety occurs in which the ocular tubercle may be conspicuously large.

APPENDIX
The primary reason for the preparation of this and previous studies of Purbeck and Wealden ostracods has been the need to find a method of correlation of these variable sequences, not only within the Wealden area, but also into Hampshire, Dorset and beyond. To this end, numerous surface exposures and boreholes have been examined in Purbeck and Wealden strata across southern England. Parts of these correlations have been published (e.g. Anderson, 1971, pl. VI), but the large format and complexity of the diagrams has meant that they are not easily reproduced for publication. They do, however, illustrate the primary evidence for the correlations derived from the identification of the ostracods whose identity and distribution are summarised above. These beds are only seen near the western shore. There is a lower sequence of mainly Portland-type sediments with Myophorella, Liostrea and ostracods belonging to the genus Macrodentina but including some less saline episodes with species of Fabanella and even, though rarely, with Cypridea spp. This is followed by more typical Lower Purbeck sediments with Fabanella as the dominant ostracod genus and with species of Cypridea rather more common. D. The stratigraphical distribution of ostracod species in the Cinder Beds.
the Purbeck and Wealden of England.
This shows the ostracod species as a percentage of the total ostracod fauna. C-phase and S-phase species are both shown (unlike Figs. 3 and 4 in this paper). The predominance of S-phase species in both the early parts of the Purbeck and latest Wealden is clearly seen. E. Sequence of ostracod faunas in the Wadhurst Clay of Sussex and Surrey. A detailed correlation between the Wadhurst Park, Westfield, Cooden, Little Maxfield, Freshfield Lane and Warlingham boreholes showing graphically the percentage of S-phase species as a percentage of the total ostracods, and the C-phase species as a percentage of total C-phase ostracods.
Note the marked faunal change between the Fairlight and Lindfield Faunicycles. F. Sequence of ostracod faunas in the Lower Weald Clay (Henfield phase) of Sussex and Surrey. A detailed correlation between the Ripe, Hailsham, Warlingham and Cuckfield boreholes and Clock House and Warnham pits showing graphically the percentage of S-phase species as a percentage of the total ostracods and the C-phase species as a percentage of the C-phase ostracods. This shows the successive replacement of C. dorsispinata and C . tuberculata of the basal Wealden by C. pumila and C . marina and these by C. clavata in the highest parts of the Weald Clay.
Explanation of Plate 1 All the specimens figured within each plate are in the collection of the British Geological Survey, at Keyworth, Nottinghamshire. Earlier citations of Mik(M) numbers often include the three number suffix 001. This is here omitted as all these specimens are alone on single cell slides. Plates, but not the figures within each plate, are ordered by first appearance of species.    (Wolburg), male, Mik(M) 3220; length 1 *14mm, height 0.665mm; Guestling No. 1 Borehole,Sussex. p. 36 Fig. 11. Cypridea peltoides peltoides Anderson (angular form, 1971, pl. 7, fig. 7  The following figures illustrate species of less stratigraphical significance than those preceding either because of their rarity or of their long range.